Field of the Invention
The invention relates to a redundant optical radiant energy source, especially for metrology applications. Further, the invention relates to a redundant optical device for creating optical radiation, especially for metrology applications.
Description of Related Art
Optical radiant energy sources are widely used in various technical fields. While in the field of data transmission optical radiation is used to transport information, in other technical fields, for example in the field of material treatment and material processing, the optical radiant energy is used to influence the material properties of a workpiece or the shape a workpiece. Further, in the field of metrology, the optical radiant energy can be used to control the internal or external degrees of freedom of particles. This application is of special interest in the field of atomic clocks. Here, the optical radiant energy is supplied to a beam of particles (atoms or molecules) in order to control either the particles' internal degrees of freedom, especially their energy state and spin, or the particles' external degrees of freedom, especially their position and velocity.
In the field of atomic clocks, optical devices for creating optical radiation at an extremely stable center wavelength and having a very narrow optical bandwidth are required. Further, such optical devices must reveal an extremely high stability in respect of the wavelength noise.
In order to guarantee the required stability, temperature-controlled narrowband lasers are used in such optical devices, for example distribution feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers. A laser chip, which is produced using the DFB or DBR technology, is usually assembled into a laser module by mounting it onto a thermally conducting laser support, for example a C-mount support, and placing a thermal sensor close to the laser chip in order to detect its temperature. The laser support or mechanical mount may then be assembled on a thermal electric cooler (TEC) in order to control the temperature of the laser chip to a desired value. Of course, as the center wavelength and other optical properties of the spectrum of the optical radiation created by a semiconductor laser are temperature-dependent, the temperature control may be effected in such a way that the temperature of the laser chip is stabilized at a desired value and thus the optical properties of the radiation created including the center wavelengths are kept constant at respective desired values.
In atomic clocks, redundant laser modules are used so as to achieve a sufficiently high instrument reliability and increase its lifetime. Only one of the laser modules is activated at a given time and automatic switching to the spare laser module in case of failure of the main laser module is implemented.
In order to provide such redundancy at the instrument level, an optical architecture for a redundant optical radiant energy source as shown in FIG. 1 is widely applied. This redundant optical radiant energy source 1 comprises two optical devices 3 and 5 for creating optical radiation in the form of laser modules 7 and 9, respectively. Each of the laser modules 7, 9 comprises a housing 11 comprising a cap 13 and a base part 15. A plurality of electrical connection pins 17 extend through the base part 15. A heating/cooling device 19 is mounted to the surface of the base part 15. Preferably, the heating/cooling device 19 is in good thermal contact with the base part 15, which may consist of a metal, for example copper or aluminum, in order to act as a heat sink if thermal energy must be transported to the outer side of the housing 11. The heating/cooling device 19 may, for example, be a thermo-electric cooler (TEC).
A laser support 21 is mounted to the heating/cooling device 19 in a thermally conducting manner. On or within the laser support 21, a laser chip 23 is provided. Further, on or within the laser port 21 a thermal sensor 25, for example a thermistor, is provided. In the embodiment according to FIG. 1, an embedded laser chip 23 and an embedded thermal sensor 25 are used. Of course, the laser support 21 also consists of a material or material combination having a high thermal conductivity. In this way, the thermal energy produced by the laser chip 23 can be transported to the heat sink realized by the heating/cooling device 19 (as the case may be, in connection with the base part 15). Vice versa, in case the laser chip 23 is to be kept at a desired temperature higher than the temperature which would arise without heating or cooling the chip, the heating/cooling device may produce thermal energy in order to stabilize the temperature of the laser chip 23.
As mentioned above, the laser support 21 may be a C-mount support. The thermal sensor 25 is preferably provided sufficiently close to the laser chip 23 so as to detect the laser chip temperature with sufficient accuracy (and without inacceptable time delay).
As apparent from FIG. 1, the laser chip 23 may be centrally provided within the laser support 21, and the laser support 21 and the heating/cooling device 19 may be centrally provided within the housing 11. The cap 13 of each housing 11 has a window 11 a (shown as a dashed line), which is transparent for the optical radiation created by the laser chip 23. In FIG. 1, the optical radiation created by each of the laser chips 23, which are comprised by the laser modules 7, 9, is shown as respective laser beams 27, 29. As mentioned above, the laser chip 23 may be a DFB or DBR laser chip. As the radiation arises from a narrow junction of a few micrometers in the semiconductor material, each of the laser beams is a highly diverging beam. The window 11a may have a size which is sufficient to transmit the whole laser beam created by the laser chip without having the function of an aperture. Of course, the window 11 a may be designed in such a way that the boundaries of the laser beams 27, 29 are defined by the shape of the windows 11a. However, it might be preferred to design the window 11a in such a way that it reveals a size larger than the laser beam cross-section in the plane or area of the window 11a in order to avoid scattering and diffraction induced by the window aperture. The laser beam shape may in this case be defined by a collimation device 35, 37 (see below).
Each of the laser beams 27, 29 has an optical axis 31, 33 which essentially coincides with the axis of the respective laser module 7, 9. The axis of the laser module 7 and the optical axis 31 of the laser beam 27 and the axis of the laser module 9 and the optical axis 33 of the laser beam 29 intersect at a right angle, wherein, in the embodiment shown in FIG. 1, the optical axis 31 is oriented horizontally and the optical axis 33 is oriented vertically.
Each of the laser beams 27, 29 is collimated by a collimation device 35, 37, wherein the optical axis of the collimation device is adjusted so that it coincides with the optical axis of the respective laser beam 27, 29. The collimation devices 35, 37 may be adjustable in a plane perpendicular to the respective beam axis 27, 29 and, if required, also in direction of the optical axis. The lateral adjustment accuracy influences the beam's homogeneity and the beam's tilt angles, while its axial adjustment accuracy determines the beam's parallelism.
As shown in FIG. 1, the diverging laser beams 27, 29 as created by the laser chips 23 of the laser modules 7, 9 are converted into essentially parallel laser beams due to the function of the collimation devices 35, 37.
The collimated laser beams 27, 29 are directed to a beam splitter 39, for example a semitransparent mirror. The beam splitter 39 may be a non-polarizing beam splitter, wherein, in this case, a common spitting ratio is 50:50, that is 50% of the available optical power of each of the laser beams 27, 29 is dropped. Of course, as mentioned above, only a selected one of the laser beams 27, 29 is present at the time as one of the laser modules 7, 9 serves as a main source and the other one serves as a redundant source. By using a polarized beam splitter, the power loss may be reduced to zero (apart from material absorption). However, both beams will be orthogonally polarized, which might not be acceptable for specific applications.
The optical arrangement shown in FIG. 1 has the advantage that it is fully configurable as there are independent laser modules, collimation devices and optical alignment means. However, this arrangement suffers from increased complexity, reduced reliability due to multiple required alignments and a significant power drop. This leads to high costs for realizing such an optical arrangement.